Lecture5

Lecture5 - Lecture 5 Applications of Secret Key...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Lecture 5 Applications of Secret Key Cryptography Message Authentication Codes and Cryptographic Hash Functions CNT 5412 Network Security
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Cryptographic hash functions • Also called message digests • Map a large space of values to a small space • Cannot be easily invertible • Examples: MD2, MD4, MD5 (128 bits), SHA-1(160 bits) SHA-2 (256/224 bits, 512/384 bits) What does it mean for a hash function to be broken? 2
Background image of page 2
3 Secret Key Cryptography: Insecure Channels and Media • Confidentiality – Using a secret key cipher such as DES/CBC, we can assure that messages sent on a medium cannot be tapped by an eavesdropper – Distribute a secret key between two parties, then use encryption on the sender’s side and decryption on the receiver’s side using any of the block or stream ciphers – Can use the same technique for storing information on a disk: encrypt the information and decrypt when needed
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4 Ciphers and Authentication • Authentication: verify each other’s identity • Strong Authentication – Proving knowledge of a secret without revealing it • Use a secret key cryptographic algorithm and a challenge / response protocol
Background image of page 4
5 Challenge-response authentication with shared secret K What is the flaw: hint suppose two channels are opened from Alice to Bob Bob Alice r A r B K(r B ) K(r A )
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
6 Challenge-response authentication with a message digest Note: this has the same operational flaw as previous It also has a flaw in the computation of the digest Bob Alice r A r B MD (K AB | r B ) MD (K AB | r A )
Background image of page 6
7 Integrity? • Need to add the analogue of a checksum that will insure that if there are any changes to the message, this will be detected • Cannot rely on checksums such as CRC because they are designed for channels with noise, but not for malicious adversaries • Adversary should not be able to compute the correct checksum for an altered message
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
8 Message Authentication Code MAC • The term message integrity code (MIC) is sometimes used and probably more accurate • A MAC is a cryptographic checksum that serves as an authenticator of the message
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 25

Lecture5 - Lecture 5 Applications of Secret Key...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online